The present invention relates to an optical disc reproducing apparatus which outputs bits corresponding to recorded information on the disc by scanning with a laser beam to reproduce the recorded information through variation in the detection levels of the bit signals.
In recent years, optical disc apparatus using laser beams have been used. In general, an optical disc may have as much as twice the record track density as a magnetic disc to compose a large capacity memory, and is characterized in that the laser beam may be focused on the medium surface of disc, the positioning of the beam being performed through non-contact by a pregroove track prior to reading information on the disc, so that the recording medium may be removable.
Also, the various known apparatus have been provided because of differences in recording and reproducing principles or differences in a light source for recording, reproducing and erasing use on a record medium. For example, in the magneto optic disc apparatus, a semiconductor laser is used as the light source for recording, reproducing and erasing information, a magnetic film is prepared by the evaporation or sputtering method on the disc as the recording medium to record information through variation in the magnetizing direction in accordance with the information data.
In the conventional magneto optic disc apparatus, the temperature of the magnetic film is locally raised to near the curie point by the application of a recording laser beam of an output level higher generally as compared to the reproducing beam, the magnetizing direction is varied by an application of the external magnetic field to effect a recording operation. In the digital information record, the reverse interval of the magnetic polarity to be formed by the use of the above-described method or the interval between the same polarities is varied in accordance with the information to be recorded.
In the optical disc apparatus, the recording conditions vary because of variations in recording sensitivity from disc to disc, in the recording position of the disc, change in the beam power of the laser element, and changes in the recording sensitivity by the ambient temperature. As a result, the window margin of the reproduction signal is lost by the variation of the recording bit length, so that a recording density taking this variation into account is required to be set, thus resulting in an obstacle to attaining a higher recording density.
Also, in the conventional data storing method used in the magneto optic disc apparatus, the recorded bit length along the disc rotating direction of the magnetized reversing portion is required to be strictly controlled. The factors for deciding the recorded bit length are the recording laser power and the recording sensitivity of the magnetic medium. The variation in recording laser power is caused by the variation in the output characteristics of the semiconductor laser through aging. Also, the recording sensitivity in the magnetic medium is difficult to be controlled in the individual optical magnetic disc and usually has a variation width. The variation in the recording laser power and the recording sensitivity of the magnetic medium may be expected by the circumstance of use, particularly the ambient temperature of the optical magnetic disc apparatus.
The changes in the recorded bit length caused by the above-described variation cause a reduction in the tolerance with respect to detection window width in the digital recording or a reduction in the signal-to-noise ratio by overlapping onto the side of the short length of the recorded bit length, thus resulting in deterioration in the data error ratio in the worst condition.